1. Field of the Invention
The present invention is in the field of non-dispersive gas analyzers of a type typically used to measure the concentrations of specified gases in automotive emissions and in breath analyzers. More specifically, the present invention relates to apparatus having no moving parts and capable of measuring the concentrations of one or more specified components in a mixture of gases.
2. The Prior Art
The term "nondispersive" as used herein refers to the apparatus used, typically a narrow-band optical or infrared filter instead of a prism or diffraction grating, for isolating for purposes of measurement the radiation in a particular wavelength band that normally coincides with a strong absorption band in the absorption spectrum of a gas to be measured.
As discussed in U.S. Pat. No. 3,811,776 to Blau, Jr., nondispersive gas analyzers typically incorporate, in addition to the narrow band filter, a gas cell containing the gas of interest, and an identical cell evacuated or filled with a gas that is transparent at the wavelength used. Typically, these cells alternately are moved into and out of a radiation beam. In another common arrangement, two beams are used and apparatus is provided for switching between the alternative beams.
Typically, both the alternative movement of the cells or switching of the beams is accomplished by rotating machinery. The use of such rotating machinery has generally been considered necessary but undesirable. Typical problems included noise, vibration, size and weight, electrical noise from the driving motor, and wear of the bearings.
Typically, these problems are compounded where the analyzer is required to measure several components.
The present inventor recognized that the problems associated with the rotating machinery of prior art gas analyzers could be overcome by employing solid-state devices to accomplish the desired switching action, thereby making possible a gas analyzer having no moving parts.
It has long been known that certain materials such as germanium, silicon, cadmium sulphide, zinc oxide, and certain glasses are characterized in that their optical transparency characteristics in terms of wavelength of light vary with the temperature of the material, as discussed in U.S. Pat. No. 2,824,235 to Hahn, Jr., et al.
In U.S. Pat. No. 4,283,113, Eden discloses the use of a thin film of a vanadium oxide, switchable between a high-reflectance state and a high-transmission state for switching a light signal between optical fibers. The thin film is alternately heated and cooled by thermoelectric junctions placed in thermal contact with the edges of the substrate on which the thin film is deposited.
In U.S. Pat. No. 4,515,472, Welch discloses the use of a scanning electron beam to produce localized heating in a vanadium dioxide layer. The localized heating produces a highly reflective small spot on the otherwise transmissive vanadium dioxide layer.
Thus, although the characteristics of vanadium dioxide and certain other thin films were known in the art, it remained for the present inventor to show that such films could advantageously be combined with certain other optical components to produce uniquely useful combinations that make possible a multicomponent gas analyzer having no moving parts.